In general, polyimide resin has a high insulation performance. Accordingly, the polyimide resin is widely applied to electronic/electric equipment and electronic components including circuit boards and printed wiring boards as parts or components requiring high reliability.
In recent years, electronic equipment is increasingly demanded to store large amounts of information corresponding to the highly-networked information society, process them rapidly, and transmit them at high speed. Accordingly, the polyimide resin used in the electronic equipment is also demanded to have higher performance, particularly a reduction in dielectric constant and a reduction in dielectric loss tangent, for electric properties corresponding to higher frequencies.
An attempt to provide a reduction in the dielectric constant of polyimide resin was proposed, for example, by Japanese Laid-open (Unexamined) Patent Publication No. 2000-44719, according to which after hydrophilic polymer is dispersed in a precursor of polyimide resin soluble in an organic solvent, the hydrophilic polymer is removed by baking it or by extraction of the solvent, to form pores in the precursor of polyimide resin to thereby produce porous polyimide resin.
When the pores are formed in the precursor of polyimide resin by removing the hydrophilic polymer, it is ideal that the form of the microphase-separated structure wherein the hydrophilic polymer is dispersed in the precursor of polyimide resin is kept unchanged. However, when the pores are formed in the precursor of polyimide resin by removing hydrophilic polymer by baking it or by extraction of the solvent, followed by imidization of the precursor of polyimide resin, the pores are flattened or clogged, so that the porosity is reduced below the ideal one, resulting in a problem that sufficient reduction of the dielectric constant cannot be obtained.
Although it is preferable that the pores has a smaller diameter to retain mechanical strength and heat resistance of the porous polyimide resin produced, when the pores are reduced to a diameter as small as 1 μm or less, there arises the specific problem that the pores may be clogged with ease.